The present invention concerns spinal fixation systems, and particularly systems utilizing elongated rods adjacent to the spinal column. More specifically, the invention concerns improvements to a device which connects two approximately parallel elongate members, such as spinal rods, to enhance rigidity of the system.
Spinal fixation systems are implanted during a surgical procedure to treat a variety of problems. These treatments include correction of congenital spinal deformities, repair of spinal injuries and fusion of vertebra to stabilize degenerative conditions and alleviate chronic lower back pain. Several techniques and systems have been developed for correcting and stabilizing the spine and facilitating spinal fusion. In one common system, a longitudinal member, such as a bendable rod, is disposed along the vertebral column and is fixed to various vertebrae along the length of the column by way of a number of fixation elements. A variety of these vertebral fixation elements can be provided, such as hooks or bone screws, which are configured to engage specific portions of the vertebra. Usually, the surgeon first attaches the vertebral fixation elements to the spine in appropriate anatomic positions, and then attaches each vertebral fixation element to the spinal rod.
Commonly, two or more rods each with a number of vertebral fixation elements are used. Typically, two nearly parallel rods are employed, one on each side of the spinous processes of the vertebral column. The TSRH.RTM. Spinal System sold by Danek Medical, Inc. is often configured in this manner. Details of the TSRH.RTM. Spinal Implant System are disclosed in the "TSRH.RTM. Surgical Technique Manual" provided by Danek Medical, Inc., published in 1990, which disclosure is incorporated herein by reference.
Existing vertebral fixation element attachment means include attachment to a clamp body which clamps on a longitudinal member with the attachment means operating independently of clamping the clamp body to the longitudinal member. Examples of such a system are shown in U.S. Pat. Nos. 5,024,213 and 4,987,892. Similarly, a clamp body for clamping to a longitudinal member may be integrally formed as part of the vertebral fixation element as disclosed in U.S. Pat. Nos. 5,147,360, 5,005,562 and 4,950,269. Numerous other methods of attaching a vertebral fixation element to a clamp body which, in turn, is clamped to a longitudinal member are known to those of ordinary skill in the art.
One improvement known in the art is to attach the vertebral fixation element by way of clamping to the longitudinal member. For example, the TSRH.RTM. Spinal System clamps the vertebral fixation elements to the rod by way of eyebolts. The eyebolts are positioned on the spinal rod and captured within yolks of the vertebral fixation elements. Specifically, FIG. 1 depicts an existing system described in U.S. Pat. No. 5,246,442 to Ashman et al. for clamping a spinal hook 100 to rod R. Eyebolt 102 is threaded on to rod R by inserting rod R through passage 103. Spinal hook 100 has a vertebra engaging portion 101 and a connection portion 104 extending from the vertebra engaging portion 101. The connection portion 104 is formed by a pair of posts 105 which define a slot 106 therebetween. Also, the posts 105 have opposing lateral surfaces 108 in the form of grooves for engaging rod R. Slot 106 is configured to receive threaded stem 109 formed on eyebolt 102. Nut 110 is configured to engage the threaded stem 109. The hook 100 is clamped to rod R by threading the nut 110 on threaded stem 109 received in slot 106 so that the nut 110 presses against the hook 100 disposed between the threaded nut 110 and the rod R. This assembly offers a 3-point shear clamp of high structural integrity.
A system disclosed in U.S. Pat. No. 5,261,909 to Sutterlin et al., includes a variable angle bone screw clamped to rod R using an eyebolt, as depicted in FIG. 2. In particular, variable angle bone screw 200 has a vertebral engaging portion 201 and a connection portion 204 extending from the vertebral engaging portion 201. The connection portion 204 is formed by a pair of posts 205 which define a slot 206 therebetween for receiving a clamping means such as an eyebolt stem. Also, the posts 205 have opposing lateral surfaces, with the preferred configuration of these surfaces including at least one opposing lateral surface 207 having a plurality of radial splines 209.
Referring to FIG. 3, a clamp assembly washer 220 is shown for use in engaging the variable angle screw 200. The Clamp assembly washer 220 has an opening 222 for passing over bar 34 with projection 33 extending therethrough. The mating surface 227 of the clamp assembly washer 220 includes a plurality of radial splines 229 for interdigitating engagement with the radial splines 209 of the splined surface 207 of the variable angle bone screw 200 shown in FIG. 2. The interdigitation of the washer splines 229 and the bone screw splines 209 facilitate rigid fixation of the variable angle bone screw 200 at non-vertical angles in relation to an attached clamp assembly when the splined surface 207 and the mating surface 227 are clamped together. Further details of this assembly are disclosed in U.S. Pat. No. 5,282,801.
Notably, some clamping assemblies require extensive access to the side of the assembly. Because surgical access is ordinarily from the posterior of the patient, this "side tightening" presents some difficulties. Specifically, the threaded stem of the eyebolt and the nut engaging the stem both project laterally away from the rod. It has been found in practice that it is often cumbersome to engage the nut with a wrench to tighten the nut onto the eyebolt assembly. Moreover, simple mechanics dictates that the wrench can only be moved through a partial turn before the handle of the wrench contacts the surrounding tissue. This necessitates taking the wrench off of the nut and re-engaging it for an additional partial rotation. Ratchet-type wrench systems are typically not acceptable in procedures of this sort because the lateral space required for the ratchet mechanism unnecessarily impinges on the surrounding tissue and requires greater space at the surgical site. Consequently, a subsequent improvement is disclosed in the U.S. Pat. No. 5,282,801 to Sherman. This improvement clamps a vertebral fixation element and rod together by way of a 3-point shear clamp assembly. Notably, this assembly is "top-tightening" because a set screw, completely accessible from the top, is used to initiate and adjust the clamping of the components.
It is a primary goal of the surgeon using a spinal implant system to obtain maximum construct rigidity. Thus, once two rods are fixed to various vertebral fixation elements along the spine, each on opposing sides of the spinous processes, a rigid transverse connection which bridges the rods is often desired. This transverse connection improves overall spinal fixation system integrity. One method used is an adjustable transverse rod to connect the two main rods as shown in U.S. Pat. No. 5,005,562 to Cotrel. One problem with this approach is that the transverse connector clamp is separate from the vertebral fixation element clamps. As a result, the transverse connector requires additional space along the length of the rod limiting the configurations possible with respect to the vertebral fixation element clamps. Also, the relative size and bulkiness of this transverse connection method complicates implantation and limits the connection options available to the surgeon. The size and bulkiness of the system is especially important because a significant portion of the patient population for spinal implant fixation systems is made up of pediatric patients. Bulky implants are not easily implanted into small, thin people because the space around the vertebra is limited. Also, in patients with severe deformities of the spinal column, the vertebral fixation sites are often severely limited, so connection flexibility is paramount. Finally, the transverse rod does not provide exclusive top-tightening ability complimentary to recent vertebral fixation element clamping improvements.
Similarly, the "TSRH.RTM. Surgical Manual", illustrates a rigid transverse bar or CROSSLINK.RTM. connecting the rods by way of eyebolt clamps. Similar to the vertebral fixation element attachment, an eyebolt is strung on each rod prior to clamping. The transverse connector receives the threaded stem of the eyebolt through an opening on opposing ends of the connector. A nut is then threaded on each stem to clamp the transverse connector between the nut and the rod at each opposing end. Although this assembly permits top-tightening at the associated clamp, it requires anticipating the placement of additional dedicated eyebolts prior to clamping the vertebral fixation elements to the rod. Also, it still suffers from the size, bulkiness and "rod crowding" constraints of other systems because it does not facilitate clamping at the same site as a vertebral fixation element.
Spinal procedures are rapidly becoming prevalent surgeries, largely because of the high incidence of low back pain. In the past, surgical techniques for alleviating low back pain or for addressing deformities or injuries has required fairly complicated and massive surgical procedures. The focus in recent times has been to greatly reduce the degree of invasion into the patient required for stabilizing a spine with instrumentation, as well as to reduce the amount of trauma to tissue surrounding the instrumentation, both during the procedure and after the spinal instrumentation has been implemented. Moreover, an implant which is easy to assemble, simple to adjust and high in rigidity substantially reduces the risk of complications adversely affecting a patient.
Consequently, there is a need for a new transverse connector which can connect at the same clamp location along the rod as any vertebral fixation element and still provide the rigidity of existing connectors, preferably offering the integrity of a 3-point shear clamp configuration. Furthermore, this new connector needs to clamp directly to the rod and be removable without disassembly of vertebral fixation elements clamped to the rod. A top-loaded device harmonizes these goals. Also, this new connector would ideally offer a clamping mechanism which is installed and adjusted independent of the vertebral fixation element clamping. Finally, the ease of adjustment inherent in a top-tightened system should be available for the transverse connector clamp, while preserving top-tightening clamp adjustment for the vertebral fixation elements.